The present invention relates to a method of producing a nickel electrode for an alkaline storage battery comprising loading a positive-electrode active material containing nickel hydroxide as its principal component into the pores of a porous electrically conductive substrate, and more specifically, a method comprising impregnating a porous electrically conductive substrate (a porous sintered nickel substrate) with an acidic nickel salt, and then performing an alkali treatment.
In order to comply with the recent demand for a rechargeable battery having a higher energy density, improvements are being made on alkaline storage batteries such as nickel-cadmium secondary batteries and nickel-hydrogen secondary batteries. The nickel electrodes that are employed in the alkaline storage batteries of the type above are produced by impregnating a porous electrically conductive substrate (a porous sintered nickel substrate) with an acidic nickel salt, and by then performing alkali treatment and the like. In this manner, a porous electrically conductive substrate can be obtained with a positive-electrode active material containing nickel hydroxide as its principal component being loaded into the pores thereof.
However, in a prior art nickel electrode comprising an active material obtained by converting nickel nitrate to nickel hydroxide, which is prepared by immersing a porous electrically conductive substrate impregnated with a nickel nitrate into an alkali, the potential of oxygen evolution at the nickel electrode falls close to the charging reaction potential of nickel hydroxide. Particularly, since the potential of oxygen evolution (i.e., the oxygen overvoltage) decreases at higher temperatures, the oxidation reaction of the nickel active material becomes competitive with the gaseous oxygen generating reaction.
Due to the lowering of the charge efficiency (the charge acceptance) that occurs as a consequence of the above phenomenon, there happened a problem of impairing the battery performance at higher temperatures. In the light of such circumstances, various methods have been proposed to improve the charge efficiency (the charge acceptance) by increasing the oxygen overvoltage. For instance, in JP-A-Hei-11-73957 is proposed to increase the oxygen overvoltage by incorporating Ni mixed together with Co and Y in the nickel electrode. In JP-A-Hei-10-125318 is disclosed a method of increasing the oxygen over voltage by providing, as a surface layer portion of the nickel electrode, an independent crystal containing an A-group element selected from Mg, Ca, Sr, etc., and a B-group element selected from Co, Mn, etc., in the form of a solid solution.
Further, in JP-A-Hei-10-149821 is proposed to increase the oxygen over voltage by a method comprising forming a surface layer containing Ca, Ti, etc., at a high concentration on the nickel electrode, while incorporating Al, V, etc., at a high concentration inside the nickel electrode. Furthermore, in JP-A-Hei10-255790 is disclosed a method of increasing oxygen over voltage by covering the surface of nickel hydroxide (Ni(OH)2) particles with a layer of a hydroxide of Ni and Y.
As described above, various methods for increasing the hydrogen overvoltage by using elements such as Ca, Sr, Y, Al, Mn, etc., have been proposed to present. Concerning the positions for adding the elements such as Ca, Sr, Y, Al, Mn, etc. in the aforementioned methods, it is advantageous that these elements are incorporated at the surface of the principal active material, i.e., nickel hydroxide (Ni(OH)2), in such a manner that these elements should be present in abundance in the vicinity of the boundary between the nickel hydroxide and the electrolyte, because the oxygen over voltage can be increased more effectively.
In case of incorporating the elements above at a higher amount in the vicinity of the boundary between the nickel hydroxide and the electrolyte, a sequential operation of first immersing the porous electrically conductive substrate into a solution of an acid salt based on nickel and then immersing the substrate into an alkaline solution after intermediate drying is repeated for a predetermined times to obtain a n active-material loaded plate electrode loaded with a desired amount of active material. Then, the active-material loaded plate electrode thus obtained is immersed in an acid salt solution containing elements such as Ca, Sr, Y, Al, Mn, etc., and after intermediate drying, the resulting active-material loaded plate electrode is immersed in an alkaline solution to form a hydroxide layer of the elements such as Ca, Sr, Y, Al, Mn, etc. on the surface of the active-material loaded plate electrode. Such an operation is preferred from the viewpoint of taking advantage of the existing production lines.
However, if the nitrate solution should be high in temperature and low in pH value in case of immersing the active-material loaded plate electrode in an acid salt solution containing the elements such as Ca, Sr, Y, Al, Mn, etc. as above, there have been found problems of lowering the capacity of the battery due to the elution of the active materials once loaded in the active-material loaded plate electrode. Furthermore, in case the loaded active material should be eluted in a large quantity, corrosion occurred on the porous electrically conductive substrate as to impair the mechanical strength of the porous electrically conductive substrate.
Moreover, the operation of loading the nickel-based hydroxide into the porous electrically conductive substrate for a desired amount, which comprises repeating for predetermined times a sequential operation of first immersing the porous electrically conductive substrate into a solution of an acid salt based on nickel and then immersing the substrate into an alkaline solution after intermediate drying, was found to cause clogging of the pores that are present on the surface of the porous electrically conductive substrate with increasing repetition of the operation. Thus, this prevented uniform impregnation of the nitrate solution containing the elements such as Ca, Sr, Y, Al, Mn, etc. into the inside of the pores that are present in the porous electrically conductive substrate, and thereby led to a problem of an insufficient exhibition of the effect of improving the charge characteristics at high temperature.
The present invention has been made with an aim to overcome the aforementioned problems. Thus, an object of the present invention is to provide a nickel electrode for alkaline storage batteries improved in the high temperature charge characteristics, and, said nickel electrode being an active-material loaded plate electrode in which, even in case it is immersed in a nitrate solution, the elution of the loaded active material is controlled and thereby prevents the battery capacity from causing a drop.
In order to achieve the above object, there is provided a method of producing a nickel electrode for an alkaline storage battery, comprising: an active-material loading step comprising preparing an active-material loaded plate electrode by loading an active-material containing nickel hydroxide as its principal component into the pores of said porous electrically conductive substrate; an immersion step comprising immersing said active-material loaded plate electrode into an impregnation solution comprising an acid salt solution (such as a nitrate solution) containing at least one element selected from the group consisting of Ca, Sr, Sc, Y, Al, Mn, and lanthanides; and an alkali treatment step comprising forming a hydroxide layer of at least one element selected from the group consisting of Ca, Sr, Sc, Y, Al, Mn, and lanthanides by immersing said plate electrode into an alkaline solution; provided that the temperature of the impregnation solution is controlled in a range of from 40 to 90xc2x0 C., and that the pH value of impregnation solution is controlled to a range of from 4 to 6.
In performing the method above, if the active-material loaded plate electrode should be immersed in an impregnation solution comprising a nitrate solution containing the elements of Ca, Sr, Sc, Y, Al, Mn, and lanthanides, which is high in temperature and/or low in the pH value, the loaded active material tend to be easily eluted in the nitrate solution. Accordingly, in order to prevent the loaded active material from being eluted from the porous electrically conductive substrate, the temperature of the impregnation solution should be set low, and the pH value of the impregnation solution should be set high. However, if the active material should be completely prevented from being eluted, a uniform permeation of the impregnation solution into inside of the pores would be prevented from occurring by the active material covering the surface of the pores that are present in the porous electrically conductive substrate. This makes it difficult to achieve the effect of forming a layer of a hydroxide of the elements of Ca, Sr, Sc, Y, Al, Mn, and lanthanides; i.e., this makes it difficult to implement a nickel electrode having excellent high temperature charge characteristics.
However, as described in the present invention, if the temperature of the impregnation solution is controlled to a range of from 40 to 90xc2x0 C., and if the pH value of the impregnating solution is controlled to a range of from 4 to 6, the active material loaded in the pores of the porous electrically conductive substrate can be eluted at a proper degree. As a result, the pores that are present in the porous electrically conductive substrate can be uniformly impregnated with the impregnation solution above deeply into the inside thereof, as to improve the high temperature charge characteristics of the porous electrically conductive substrate. In this manner, a nickel electrode for alkaline storage batteries having a high battery capacity and yet improved in high temperature charge characteristics can be implemented.